A(1,3) interaction and conformational energy of axial–axial 1,3-dimethyl interaction

Abstract

A(1,3) type strain was demonstrated directly from the nuclear magnetic resonance studies of the acyl and nitroso derivatives of 2-methylpiperidine (2 series) and 2,6-cis-dimethylpiperidine (1 series). In these two series, the bulky 2-methyl groups are forced to assume the axial conformation due to a severe A(1,3) interaction in the equatorial conformation. The free energy of activation (ΔG*) for the internal rotation around the N—C and N—N bonds in 1a–1g was determined. The comparison of these (ΔG*) values with those of the corresponding derivatives of dimethylamine indicates the decreases are in the range of 3.6–4.7 kcal/mole which is in fair agreement with the conformational energy of 1,3-diaxial CH3/CH3 interaction. An argument was presented to show that this interaction energy corresponds to the decrease of ΔG* in 1a-1g.